The symptoms of schizophrenia are generally divided into three categories; positive, negative and cognitive. Positive symptoms include hallucinations, delusions and disorganized behavior while negative symptoms are characterized by a lack of pleasure and/or interest in life. Cognitive deficit includes difficulties in the organization of thoughts and prioritization of tasks. Patients with bipolar disorder generally display circular mood changes ranging from severe depression to severe mania with or without psychotic features. Schizophrenia and bipolar disorder are among the most severe forms of psychiatric disorders that elicit overlapping cognitive deficits (Tasman et al., Psychiatry, West Sussex, John Wiley & Sons, Ltd., Second Edition, Volume 1, 2003, pp 254-272; and Sadock and Sadock, Kaplan and Sadock's Comprehensive Textbook of Psychiatry, 7 ed., Vol. 1, 2005, Philadelphia, Pa.; Lippincott Williams & Wilkins, pp 236-272 and 1330-1395) and they tend to be chronic/progressive. In contrast to positive symptoms, the negative and cognitive symptoms of schizophrenia are thought to have a greater impact on long-term disability, treatment outcome and functional recovery (Addington and Addington, 1993; Green, 1996), Dissatisfaction with therapy is attributed to lack of efficacy or intolerable and unacceptable side affects. The side effects have been associated with significant metabolic, extrapyramidal, prolactic and cardiac adverse events. See, Lieberman et al., N. Engl. J. Med. (2005) 353:1209-1223.
While multiple pathways are believed to be involved in the pathogenesis of schizophrenia leading to negative and cognitive symptoms, much attention has focused on reduced dopamine neurotransmission in the prefrontal cortex (Weinberger, 1987; Weinberger et al., 1988; Akil et al., 1999). Evidence for reduced dopamine neurotransmission in the prefrontal cortex is supported by reduced regional cerebral blood flow or hypoactivation of the dorsolateral prefrontal cortex in schizophrenia patients (Weinberger et al., 1988; Daniel et al., 1991; Okubo et al., 1997; Abi-Dargham et al., 2002). Schizophrenia related prefrontal deficits, independent from treatment or psychotic state, have been correlated with poor performance in tasks of executive function (e.g. n-back or Wisconsin Card Sorting Test) that evaluate prefrontal engagement (Weinberger et al., 1986, 1988; Carter et al., 1998; Callicott et al., 2000; Barch et al., 2001). In addition to deficits in executive function, reduced dopamine neurotransmission in the prefrontal cortex is involved in several brain activities including; attention, hedonic activities, natural rewards, and biologic activities such as cell signaling. Therefore, a compound which selectively enhances dopamine neurotransmission within the prefrontal cortex may have therapeutic potential for the treatment of cognitive and negative symptoms.
Dopamine levels in the brain are determined by biosynthesis and release, as well as its rate of diffusion, reuptake, and degradation. Catechol-O-methyltransferase (COMT), is an important enzyme involved in the breakdown of dopamine in the cortex. COMT converts dopamine to 3-methoxytyramine and the dopamine metabolite dihydroxyphenylacetic acid (DOPAC) to homovanillic acid (HVA) (Boulton and Eisenhofer, 1998). In fact, COMT acts on a variety of biogenic catecholamines as well as catecholestrogens, dietary phytochemicals and ascorbic acid. In subcortical structures (e.g. striatum), dopaminergic signalling is primarily regulated by removal of dopamine from the synaptic cleft via rapid uptake by the dopamine transporter (DAT) and/or norepinephrine transporter (NET). Regulation of dopamine transmission in the prefrontal cortex is markedly different. DAT is less densely expressed in synapses within the prefrontal cortex where dopamine is eliminated by uptake through the NET, diffusion, or metabolism by COMT and monoamine oxidase (Mazei et al., 2002; Moron et al., 2002; Lewis et al., 2001; Sesack et al., 1998; Smiley et al., 1994). COMT inhibitors would therefore be predicted to selectively increase cortical dopaminergic signaling and thereby improve cognitive function.
The COMT gene is located in the chromosome 22q11.21 region which has been linked to schizophrenia, bipolar disorder, ADHD and substance dependency (Williams, et 2003 and Takahashi et al., 2003). There are two major isoforms of COMT, membrane-bound COMT (MB-COMT) is the predominant form involved in the degradation of synaptic frontal lobe dopamine in human brain (Lachman et al., Pharmacogenetics (1996). 6(3):243-250). The other form is soluble COMT (S-COMT) which is transcribed from a different promoter than MB-COMT and is otherwise identical to human MB-COMT minus 50 amino acids at the N-terminus of the protein. In humans, COMT activity is modulated by a single nucleotide polymorphism at Val158Met (MB-COMT). Due to differences in enzyme thermostability, homozygous Met carriers have lower COMT activity, heterozygotes exhibit intermediate activity and homozygous Val carriers have greater enzyme activity (Chen et al., 2004). Despite the differences observed in activity based on genotype, only a modest relationship between Val158Met genotype and cognitive performance has been demonstrated by meta-analysis in normal individuals, while no effect was observed in schizophrenia. Based on an inverted-U relationship thought to exist between dopamine receptor activation and prefrontal cortical functioning, these findings might be reconciled with the fact that disease state, along with multiple environmental and genetic factors, contribute to prefrontal efficiency and dopamine levels (reviewed in Tunbridge et al., Biol Psych, 2006).
Although clozapine, Zyprexa, Risperdal and other anti-psychotic drugs have been useful for the treatment of positive and arguably the negative symptoms of schizophrenia and bipolar disorder, they have not been free from side effects such as agranulocytosis, sedation, weight gain, hyper-lipidemia and hyperglycemia, all of which limit their applications (Tasman et al., 2003; Sadock and Sadock 2005). Thus, there remains a need for medications that effectively treat negative symptoms and cognitive deficit, have no major side effects, and are effective in the treatment of schizophrenia, bipolar disorder, depression, substance dependency, and ADD/ADHD, etc. Such medications might also be used to reduce such symptoms when they occur as part of another psychiatric syndrome or when they are incidental to a neurological disorder.